This invention relates to viscous conjugated diene block copolymers. Further, the invention relates to crosslinked epoxidized polymers made from such block copolymers and adhesive compositions made therefrom.
Curing of adhesives based on conjugated diolefins and, optionally, vinyl aromatics has increased the range of service properties for such adhesives. Radiation curing and chemical curing of polymers to make such adhesives is known. This curing causes covalent crosslinking of the polymerized conjugated diolefins which is evidenced by a high gel content of the crosslinked polymer. Before crosslinking, the polymers are melt processable but after crosslinking, the gel cannot be processed as melts. Crosslinking therefore enhances solvent resistance and improves elevated temperature shear properties. Compositions can therefore be applied to a substrate in a melt and then crosslinked to form a superior adhesive. However, improvements in the adhesives could be made if the adhesives could be processed at lower temperatures and cured at lower dosages of radiation.
Further, the known curable adhesives which are based on vinyl aromatics and conjugated diolefins do not have particularly good long term heat, weather and ultraviolet stability due to the need to utilize unhydrogenated polymers. The known vinyl aromatic-conjugated diolefin based adhesives which are curable are unhydrogenated polymers. Hydrogenation is known to improve long term heat, weather and ultraviolet stability, but it removes the double bonds which are needed to effect the curing by radiation crosslinking. The requirement for this unsaturation is particularly evident when typical tackifiers are present in the compositions because their presence generally inhibits crosslinking of the polymer.
It is an object of the present invention to provide a block copolymer which can be modified so that it can be crosslinked, preferably by radiation, and which is easily melt processable before crosslinking but has a high gel content after crosslinking. Another object is to obtain a material which flows easily and does not have a strong elastic component prior to use. Further, it is an object of this invention to provide an adhesive composition which is based on this crosslinkable block copolymer.